7. Project Abstract Obsessive-compulsive disorder (OCD) is a chronic and disabling condition affecting over 3 million Americans in the US. The negative impact of OCD is heightened by the fact that the large majority of individuals with OCD (Up to 65%) never receive treatment,1 and of those that do receive treatment, a significant proportion (approximately 25 to 30%) fail to benefit from empirically supported interventions.2 Thus, our efforts to understand the nature and course of OCD are ongoing. Recently, research has documented a link between circadian influences on the timing of sleep and severe OCD.3-5 These studies have documented elevated rates of delayed sleep phase disorder (DSPD; up to 42%), a circadian rhythm sleep disorder characterized by the inability to fall asleep and wake up until later than is desired or socially acceptable, in inpatient sample of individuals with OCD. While evidence suggests comorbidity among OCD and DSPD, it is currently unclear as to how endogenous biological rhythm disruptions are related to symptoms of OCD or how the presence of circadian rhythm disruption may influence the severity and impairment associated with these symptoms. The current study will extend these findings by integrating circadian rhythms theory and methodology with the study of OCD by directly measuring circadian rhythms in individuals diagnosed with OCD as well as investigating the relationship between circadian rhythm disruptions and OCD symptoms severity. Specifically, Aim One will document circadian phase preferences, endogenous biological melatonin rhythms, and behavioral indicators of circadian rhythms in individuals with OCD and the relation of these circadian parameters to severity of OCD symptoms. We hypothesize that circadian parameters will be positively associated with OCD symptom severity. Aim two will test the specificity of the relation of circadian parameters and OC symptoms by controlling for 1) other sleep parameters and 2) negative affect. It is predicted that this relation will remain significant when controlling for sleep and negative affect. Finally, Aim three will explore the hypothesis that greater daily intra-individual variability in circadian patterns will predict more severe OCD symptoms. The results of the proposed study will serve as a step toward clarifying the role of circadian rhythms in OCD and could represent a novel direction in understanding the nature of OCD.